Q. Okay, okay, I'll start using the corn gluten meal you're always talking about on my lawn. Now, in the spring, you say to put it down when the forsythia bushes begin to bloom. But how will I know the best time to put it down in the fall? Thanks!

---Dan in Lansdale, PA

What's the best time to apply corn gluten for fall weed prevention? I know the rule of thumb for spring is when the forsythia bloom, but I don't recall the time frame for the fall.

---Diana in Northern Virginia

A. That might be due to the fact that I don't think I've ever recommended a weed suppression time frame for the Fall, Diana! When I mention corn gluten at this time of year, it's typically as an excellent choice for the big fall feeding that cool-season lawns like bluegrass and fescue need to recover from summer heat. I've really only discussed corn gluten's ability to prevent weed seeds from sprouting in the Spring, when its natural pre-emergent herbicidal activity can prevent the crabgrass seed lurking in your turf from successfully germinating into America's most despised lawn weed.

But these questions got me thinking about chickweed—a 'winter annual' that springs up when most other plants are going dormant and that sometimes gets pesky in lawns. So I emailed our resident turfgrass (and corn gluten) expert, Iowa State University's Dr. Nick Christians—the researcher who pioneered the use of corn gluten as an all-natural 'weed and feed'—to see if chickweed or any other late season weeds were about to germinate.

His response floored me. He wrote back: "Actually, many broadleaf weed seeds germinate right after the summer heat stress period is over. Dandelion, clover and plantain are the main ones. Then the winter annuals like henbit and common chickweed usually germinate a little later."

Oof. Guilty as charged. Well, it's nice to know that I still have new things to learn.

So, Nick—what's the ideal timing here?

"To control dandelion, plantain and clover in a cool season turf, I would apply the corn gluten pretty much as soon as the summer heat stress is over—when daytime temperatures are no longer reaching into the 90's. For some people, that could be as early as August 15th. September 1st would be fine for most areas, and applying it a little later that than that would still catch some of the winter annuals. Just be aware that corn gluten will have no effect on existing weeds—just the seeds that would have sprouted the next generation.

"Let's use dandelion as an example of how this works. All the puffball seeds that were spread in the Spring have been lying dormant over the summer. When the weather cools down, the seeds germinate and form a little rosette—a very flat, low to the ground plant part from which the big obvious dandelion springs up when winter is over. In our test plots here at Iowa State, we saw remarkable control of dandelion—and clover—with early Spring and late summer feedings of corn gluten meal. But you have to be both diligent and patient; it typically takes around three years to see a noticeable difference, and around five years for the area to be weed-free."

And it's like crabgrass, right? If you miss the application window and the weed seeds do germinate, the corn gluten will just feed those plants as well as the lawn…'

"Yes and no," says Nick. "That is pretty much the case with crabgrass in cool season lawns in the Spring, because the turf grasses in those lawns are about to get knocked back hard by the summer heat, while the crabgrass is going to get stronger as it gets hotter. But in the Fall, cool season grasses like fescue and bluegrass are gaining strength, and can out-compete some of these weeds when both are fed—especially if the grass was in decent shape at the end of summer. So there's no disadvantage to feeding corn gluten even in late September or so.

Now, in my own defense here, I have talked about how dandelion and clover virtually disappeared in those Iowa State test plots after a few years without any kind of direct action against them. But I always thought that it was simply because the corn gluten was providing the kind of slow-release natural nitrogen that helps turf grass out-compete weeds. And the fact that those test plots were always cut at the right height and watered correctly…

"It's a combination of factors, for sure," says Nick. "A healthy, well-managed turf will outcompete quite a few of those weeds. But stopping a lot of their seeds from germinating can give you a big leg up."

Thanks Nick—I'll just add the requisite warning that because corn gluten prevents the germination of all seeds, you can't use it and then sow grass seed successfully. You'd have to get the new grass up and growing for a week or two before you feed the new lawn with corn gluten.